The same things apply when X is a virtual machine running on the host A. This way if the packet was really destined to X, A will decrement the TTL, will see that it's zero and will drop it before sending it to the computer. Similarly, when the ISP must forward a packet to the host A, it sets the TTL to 1. The ISP will detect that an unexpected TTL was received (63, 127, etc.) and will drop the packet. This way, if there's another computer X behind the one that is connected A, its TTL will be decremented by the A when sending it on the internet.
Using this information, some ISPs decided to limit the number of computers connected on a network by discarding packets with that originate from the host and have a TTL different from what they expect (64, 128, etc.) and to change the TTL of all packets destined to the host to 1. TTLs start at various numbers (see ) usually 128 or 64. When the TTL gets to zero, the IP packet gets discarded. TTL is a number associated with every IP packet that is decremented at every hop encountered on it's way. Some ISPs check the value of the TTL field of the IP packets coming from your computer. Note: For users behind a proxy server, you will have to specify your proxy settings in File -> Preferences -> Proxy. Bridging a virtual machine can now be done entirely from the graphical user interface. Version 2.1 has reimplemented some of its networking. Read below for more sophisticated setups. VirtualBox 2.0 and onwards provides straightforward networking allowing IP address assignment and internet access using NAT, the default network option.
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